energies

Article

Phenol-Formaldehyde Resin-Based Carbons for CO2Separation at Sub-Atmospheric Pressures

Noelia Álvarez-Gutiérrez †, María Victoria Gil †, María Martínez †, Fernando Rubiera †

and Covadonga Pevida *,†

Instituto Nacional del Carbón, INCAR-CSIC, Apartado 73, 33080 Oviedo, Spain; noeag@incar.csic.es (N.A.-G.);victoria.gil@incar.csic.es (M.V.G.); mariamf.mmf@gmail.com (M.M.); frubiera@incar.csic.es (F.R.)* Correspondence: cpevida@incar.csic.es; Tel.: +34-985-11-90-90 (ext. 319); Fax: +34-985-29-76-62† These authors contributed equally to this work.

Academic Editor: Wei-Hsin ChenReceived: 16 December 2015; Accepted: 6 March 2016; Published: 11 March 2016

Abstract: The challenge of developing effective separation and purification technologies that leavemuch smaller energy footprints is greater for carbon dioxide (CO2) than for other gases. In additionto its involvement in climate change, CO2 is present as an impurity in biogas and bio-hydrogen(biological production by dark fermentation), in post-combustion processes (flue gas, CO2-N2) andmany other gas streams. Selected phenol-formaldehyde resin-based activated carbons preparedin our laboratory have been evaluated under static conditions (adsorption isotherms) as potentialadsorbents for CO2 separation at sub-atmospheric pressures, i.e., in post-combustion processes orfrom biogas and bio-hydrogen streams. CO2, H2, N2, and CH4 adsorption isotherms at 25 ˝C and upto 100 kPa were obtained using a volumetric equipment and were correlated by applying the Sipsmodel. Adsorption equilibrium was then predicted for multicomponent gas mixtures by extendingthe multicomponent Sips model and the Ideal Adsorbed Solution Theory (IAST) in conjunction withthe Sips model. The CO2 uptakes of the resin-derived carbons from CO2-CH4, CO2-H2, and CO2-N2

at atmospheric pressure were greater than those of the reference commercial carbon (Calgon BPL).The performance of the resin-derived carbons in terms of equilibrium of adsorption seems thereforerelevant to CO2 separation in post-combustion (flue gas, CO2-N2) and in hydrogen fermentation(CO2-H2, CO2-CH4).

Keywords: CO2 separation; adsorption; phenolic-resin; activated carbons

1. Introduction

The sustainable production of energy is currently a topic of extensive research with the aim ofproducing low carbon footprint processes and mitigating the CO2 impact [1]. As a sustainable and cleanenergy source with minimal or zero use of hydrocarbons, hydrogen is a promising alternative to fossilfuels [2]. Hydrogen can be generated by thermochemical, electrochemical or microbial fermentationprocesses. Biological methods are preferable to chemical methods because they offer the possibility ofusing sunlight, CO2 and organic wastes as substrates for environmentally benign conversions undermoderate conditions [3]. The biological methods for generating H2 include light-dependent methods,such as direct and indirect biophotolysis and photo fermentation; and methods not dependent on light,such as the dark fermentation process and bioelectrochemical systems (BES). Hydrogen productionthrough dark fermentation has advantages over the other processes because it does not require theinput of external energy and it offers the possibility of rapid scalability owing to its similarity to thewell-known anaerobic digestion process [4]. In addition, bio-hydrogen production from anaerobicfermentation is linked to bio-methane generation since the process develops in two main stages. In thefirst stage acidogenic bacteria break down substrates into primarily H2, acetic acid, and CO2, and in

Energies 2016, 9, 189; doi:10.3390/en9030189 www.mdpi.com/journal/energies

Energies 2016, 9, 189 2 of 17

the second stage methanogenic bacteria convert acetic acid, H2, and CO2 to methane gas. The tworeactions can be separated, at least partially, in distinct bioreactors placed in series. Both streams mustbe further purified to obtain bio-hydrogen and bio-methane with a high calorific value, appropriatefor industrial applications [5]. Concentrations of H2 of up to 30–70 vol% on average and of up to45–80 vol% in the case of CH4 on average can be achieved but the presence of CO2 in both streams(20–70 vol%) is a nuisance and needs to be removed for further utilization of H2 and CH4 [6].

On the other hand, conventional energy production from fossil fuels is one of the main sourcesof anthropogenic CO2. The separation of CO2 from flue gas (post-combustion CO2 capture), is verychallenging given that it is at near atmospheric pressure and CO2 only represents between 4 to 15 vol%,depending on the fuel used, and if dry or wet basis conditions are considered. However, the conditionsrequired for CO2 transport, storage or subsequent use as a chemical make a concentration stepnecessary to reach the set target of ca. 95 vol%.

Different technologies have been evaluated to separate CO2 under sub-atmospheric pressureand particular emphasis has been paid in recent years to post-combustion CO2 capture due toincreasing concerns about global warming and climate change. Adsorption with low temperaturesolid sorbents has the potential to reduce the energy penalty associated with the separation ofCO2 at sub-atmospheric pressures, not only under post-combustion capture conditions (CO2/N2

separation) but also for bio-hydrogen (CO2/H2 separation) and bio-methane production (CO2/CH4

separation) [7,8]. However, the success of these approaches depends on the development of adsorbentswith a high selectivity, sufficient adsorption capacity and easy regeneration. Carbon adsorbentsfulfil all these requirements: they have a significant adsorption capacity at atmospheric pressureand the necessary characteristics for the separation of CO2 at sub-atmospheric pressures includinghydrophobicity, low cost and low intensity in terms of the amount of energy required for regeneration.

The distinction between activated or porous carbons and carbon molecular sieves has not beenclearly defined. Whereas most of the pores in carbon molecular sieves are in the molecular size range,activated carbons may also have very small pores. The main difference is that activated carbonsseparate molecules through differences in their adsorption equilibrium whereas carbon molecularsieves provide molecular separations based on rate of adsorption, allowing the separation of moleculeson the basis of their size, rather than on the differences in adsorption capacity [9,10].

The use of polymeric precursors to produce activated carbons allows the incorporation of customtailored features (e.g., pore size, conformation, etc.) along with other material properties to meet the requiredspecifications. Phenol-formaldehyde resin-derived microporous activated carbons have been previouslystudied in our laboratory [6,11,12]. They can be produced in a wide variety of physical forms, at a low costand with a close control of porosity. More importantly, all these features are accompanied by a very lowlevel of impurities and good physical strength, key parameter in the design of an adsorption process [13,14].

In this work, the performance of phenol-formaldehyde resin-based activated carbons prepared inour laboratory to separate CO2 at sub-atmospheric pressures has been evaluated in a fundamental studyof the equilibrium of adsorption of CO2, N2, H2 and CH4. A preliminary CO2 uptake screening testhas been carried out to discard the samples with less than 2 mol¨ kg´1 of CO2 adsorbed at atmosphericpressure. Adsorption models are very useful as they can predict the behaviour of the equilibrium ofadsorption over a wide range of temperatures and pressures. In order to predict multicomponentadsorption from binary CO2-CH4, CO2-H2, and CO2-N2 mixtures, the extended multicomponent Sipsequation and the Ideal Adsorbed Solution Theory (IAST) in conjunction with the Sips model havebeen used and the results have been compared. Both equations deliver different conclusions regardingthe heterogeneity of the adsorbate-adsorbent system. Additionally, a commercial activated carbon(Calgon BPL) was evaluated under the same operating conditions as a reference carbon adsorbent.

2. Materials and Method

Porous carbons can be prepared from a wide range of carbon-containing materials includingcoal, lignin, wood, peat and synthetic and natural polymers. Carbonisation and activation of these

Energies 2016, 9, 189 3 of 17

materials under appropriate conditions yield carbons with a rather uniform pore structure consistingof micropores whose size is similar to that of the adsorbed molecules. For instance, low-densitymaterials can be prepared by carbonisation of polymeric compounds pretreated by various procedures,including modification with inorganic additives [15]. In particular, carbon sorbents with exactly theprescribed structure have been prepared using NaCl [16] and silicon [17] additives.

2.1. Synthesis of Carbon Precursors

Phenol-formaldehyde (PF) resins were used in this work to produce carbon-based microporousmaterials. Synthesis was conducted following two different routes, acid (hydrochloric acid, 37 wt%HCl solution) and basic (sodium hydroxide, NaOH) catalysis for the Novolac and the Resol resins,respectively. Details on the synthesis of the PF resins can be found elsewhere [18]. Carbon precursorswere prepared from the aforementioned resins, and potassium chloride (KCl) was incorporated asinorganic additive. One set of materials was impregnated with a saturated KCl solution at ambienttemperature (set a) while another was boiled in a saturated KCl solution (set b). Another carbonprecursor was prepared by mixing PF resin with an agricultural by-product, olive stones (OS), at aproportion of 20:80 wt% of PF to OS. A curing agent, hexamethylenetetramine (hexa) was added to PFat a hexa ratio of 7:2 wt%. Table 1 summarizes the materials and conditions for the production of thePF resin-derived carbon precursors.

Table 1. Departing materials and conditions for the production of PF resin-derived carbon precursors.

Precursor Type of PF Resin F/P * Curing (Tmax, ˝C) Additive Incorporation K/R **

ReKCla Basic catalysis (Re) 2.5:1 80 Ambient KCl 1.7ReKClb Basic catalysis (Re) 2.5:1 80 Boiling KCl 1.9No1KCla Acid catalysis (No1) 1:1.2 100 Ambient KCl 2.1No1KClb Acid catalysis (No1) 1:1.2 100 Boiling KCl 1.7No2OS Acid catalysis (No2) 1:1 170 OS (20:80) and hexa (7:2) —

* F/P: formaldehyde to phenol molar ratio; ** K/R: potassium chloride to resin weight ratio.

2.2. Production of Microporous Carbons

The prepared carbon precursors were first carbonised and then activated with CO2 to developtheir microporous structure, preferably in the narrow microporosity domain (pore sizes of lessthan 1 nm) since CO2 adsorption at sub-atmospheric pressures is known to focus on the narrowmicropores [15,19,20].

Carbonisation was conducted in a tubular furnace under an inert atmosphere (50 mL¨min´1

of N2). Two different temperature programs were selected for PF-KCl; the two precursors ofset a were subjected to a one-step carbonisation procedure at 600 ˝C for 1.5 h while for set b,a three-step slow carbonisation treatment at 200 (1 h), 600 (0.5 h) and 1000 ˝C (0.5 h) was carried out.The PF-OS composite was carbonised at 1000 ˝C (0.2 h) by slowly heating from ambient temperatureat 4 ˝C¨min´1.

Activation with carbon dioxide favours the development of microporosity [21]. The largestincrease in porosity is produced in the early stages of the activation process. Burn off (i.e., degree ofactivation) increases the widening of existing micropores but external burning of the particle outweighsthe creation of new porosity, resulting in a net destruction of porosity, especially above 40%–50% burnoff [22]. Control of the activation parameters (temperature and burn off degree) is crucial for tailoringthe micropore size distribution. Previous works carried out in our research group have demonstratedthe relationship between the volume of narrow micropores of less than 0.7 nm and CO2 uptake atatmospheric pressure [20]. Response surface methodology was performed to obtain the optimumconditions of activation with CO2 that maximise CO2 adsorption at atmospheric pressure for eachprecursor (details can be found in [18]). Activation with CO2 was conducted in a tubular furnacewith a CO2 flowrate of 10 mL¨min´1. Table 2 summarises the conditions of the carbonisation and

Energies 2016, 9, 189 4 of 17

activation with CO2 steps. Yields of char production during carbonisation were around 40% for PF-KClprecursors independently of the carbonisation temperature selected or type of PF resin (acid, No, orbasic, Re), and slightly lower for PF-OS due to the high volatile matter content of the biomass: olivestones. Higher activation temperatures were required for the No1KCl than for the ReKCl precursorsand even higher ones for No2OS. The burn off degrees ranged between 10 and 50%, the two limits ofthe experimental region tested, for the ReKCl precursors while intermediate values were found to beoptimal for the No1KCl and No2OS precursors. It should be noted that for precursors No1KClb andNo2OS the burn off was not statistically significant, according to the response surface methodology.Consequently, the lowest burn off to yield the highest experimental CO2 uptake was selected. For thesense of clarity and simplicity the activated samples will be denoted hereafter as follows: RKa-A71(ReKCla activated carbon), RKb-A75 (ReKClb activated carbon), NKa-A82 (No1KCla activated carbon),NKb-A82 (No1KClb activated carbon) and NOS-A94 (No2OS activated carbon).

Table 2. Conditions of carbonisation and activation with CO2 for each precursor.

ActivatedMaterial

PrecursorCarbonisation Activation

Temp. (˝C) Yield (%) * Temp. (˝C) Burn off (%) **

RKa-A71 ReKCla 600 40 694 10RKb-A75 ReKClb 1000 39 722 50NKa-A82 No1KCla 600 37 810 22NKb-A82 No1KClb 1000 40 800 20NOS-A94 No2OS 1000 32 940 38

* Char yield has been estimated as yield (%) = (mc/mi)ˆ 100, where mc is the mass of char produced and mi is themass of raw precursor; ** Burn off or degree of activation is estimated as burn off (%) = (mc ´ ma) ˆ 100/mc,where mc is the mass of char and ma is the mass of activated sample produced.

3. Results and Discussion

3.1. Chemical and Textural Characterisation

The elemental composition (carbon, hydrogen and oxygen contents) of the PF resins, the PF-KCland PF-OS precursors and the corresponding chars and activated materials were determined using aLECO CHN-2000 analyzer. The acid/basic character of the carbon surfaces was evaluated by means ofthe Point of Zero Charge (pHPZC), as determined by mass titration. The data are presented in Table 3.

Table 3. Chemical characteristics of the materials.

SampleElemental Composition (wt%, daf) pHPZC

C H O *

Resol-based series

Re 74.7 5.9 19.4 7.2ReKCla char 94.7 2.2 3.1 8.5RKa-A71 96.2 1.3 2.5 9.5ReKClb char 91.0 0.8 8.2 9.4RKb-A75 92.7 1.1 6.2 9.6

Novolac-based series

No1 82.6 6.0 11.4 5.2No1KCla char 96.5 0.5 3.0 4.6NKa-A82 99.0 0.4 0.6 8.7No1KClb char 97.0 0.4 2.6 7.6NKb-A82 99.0 0.6 0.4 8.5No2 78.1 5.8 16.1 4.8No2OS char 96.3 0.8 2.9 7.4NOS-A94 98.0 0.4 1.6 8.4

daf: dry ash free basis; * calculated by difference.

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The synthesised resins, Re, No1 and No2, do not contain nitrogen and so it has not been includedin the table. As expected for a carbon precursor the C contents in the initial resins are high (>70 wt%)and are enhanced during the subsequent steps of carbonisation and activation. The resultant activatedmaterials present C contents well above 90 wt% at the expense of a reduction in the volatile mattercontent, here represented by the O and H contents.

There is a basification effect on the carbon surfaces as a consequence of the activation withCO2. This effect is particularly significant in the case of the activated carbons produced from the acidcatalyzed resins (No1 and No2). Basicity might be ascribed to basic oxygen functionalities incorporatedfrom the activating gas (CO2) or to Lewis type basic sites associated to the carbon structure [11].

Textural characterisation of the samples mainly involved the evaluation of the microporosity(pore sizes < 2 nm), given the critical role of these pore sizes on the CO2 adsorption capacity of thesamples. The N2 and CO2 adsorption isotherms at ´196 and 0 ˝C, respectively (Figure 1a,b), weredetermined in a Micromeritics TriStar 3000 volumetric apparatus. Prior to any measurements, thesamples were outgassed overnight at 100 ˝C under vacuum.

Energies 2016, 9, 189 5 of 18

Table 3. Chemical characteristics of the materials.

Sample Elemental Composition (wt%, daf) pHPZC

C H O *

Resol-based series

Re 74.7 5.9 19.4 7.2

ReKCla char 94.7 2.2 3.1 8.5

RKa-A71 96.2 1.3 2.5 9.5

ReKClb char 91.0 0.8 8.2 9.4

RKb-A75 92.7 1.1 6.2 9.6

Novolac-based series

No1 82.6 6.0 11.4 5.2

No1KCla char 96.5 0.5 3.0 4.6

NKa-A82 99.0 0.4 0.6 8.7

No1KClb char 97.0 0.4 2.6 7.6

NKb-A82 99.0 0.6 0.4 8.5

No2 78.1 5.8 16.1 4.8

No2OS char 96.3 0.8 2.9 7.4

NOS-A94 98.0 0.4 1.6 8.4

daf: dry ash free basis; * calculated by difference.

The synthesised resins, Re, No1 and No2, do not contain nitrogen and so it has not been

included in the table. As expected for a carbon precursor the C contents in the initial resins are high

(>70 wt%) and are enhanced during the subsequent steps of carbonisation and activation. The

resultant activated materials present C contents well above 90 wt% at the expense of a reduction in

the volatile matter content, here represented by the O and H contents.

There is a basification effect on the carbon surfaces as a consequence of the activation with CO2.

This effect is particularly significant in the case of the activated carbons produced from the acid

catalyzed resins (No1 and No2). Basicity might be ascribed to basic oxygen functionalities incorporated

from the activating gas (CO2) or to Lewis type basic sites associated to the carbon structure [11].

Textural characterisation of the samples mainly involved the evaluation of the microporosity

(pore sizes < 2 nm), given the critical role of these pore sizes on the CO2 adsorption capacity of the

samples. The N2 and CO2 adsorption isotherms at −196 and 0 °C, respectively (Figure 1a,b), were

determined in a Micromeritics TriStar 3000 volumetric apparatus. Prior to any measurements, the

samples were outgassed overnight at 100 °C under vacuum.

(a) (b)

0

50

100

150

200

250

300

350

400

0 0.2 0.4 0.6 0.8 1

N2

volu

me

ad

sorb

ed

(cm

3g-1

, STP

)

P/P0

RKa-A71RKb-A75NKa-A82NKb-A82NOS-A94

0

20

40

60

80

100

120

140

0 0.01 0.02 0.03

CO

2vo

lum

e a

dso

rbe

d (

cm3

g-1, S

TP)

P/P0

Figure 1. N2 adsorption isotherms at ´196 ˝C (a) and CO2 adsorption isotherms at 0 ˝C (b) for theactivated carbons tested (bold symbols represent adsorption and empty symbols desorption).

The five tested activated carbons exhibited distinctive textural features, as can be observed inFigure 1a. RKa-A71 is the least developed in terms of total porosity but shows a type I isotherm,according to the IUPAC classification, characteristic of strictly microporous materials. RKb-A75presents a wider microporosity, characterised by a more open knee at low relative pressures, and italso shows a hysteresis loop at relative pressures above 0.4, which suggests the presence of somemesoporosity in the sample. On the other hand, NKa-A82 and NKb-A82 display similar type Iisotherms but with different volumes of adsorbed N2 along the evaluated pressures. NOS-A94 is thesample that has the most extended textural development, particularly in the domain of microporosity.Regarding CO2 adsorption at 0 ˝C (Figure 1b), the uptake of the activated carbons follows the order:NKa > NKb«NOS > RKa > RKb. RKa-A71 is the sample with the narrowest microporosity as reflectedby the more pronounced curvature of the CO2 isotherm.

The micropore volumes (W0) and average micropore widths (L0) were determined by applyingthe Dubinin–Radushkevich (DR) relation [23] and the Stoeckli-Ballerini relation [24] to these isotherms,respectively. This allowed the total microporosity and the narrow microporosity (pore sizes <1 nm)

Energies 2016, 9, 189 6 of 17

to be assessed. The textural parameters estimated from the experimental N2 and CO2 isotherms arecollected in Table 4.

Table 4. Textural parameters evaluated from the N2 and CO2 adsorption isotherms at ´196 and0 ˝C, respectively.

Parameter RKa-A71 RKb-A75 NKa-A82 NKb-A82 NOS-A94

Vp (cm3¨ g´1) 0.08 0.46 0.45 0.32 0.53W0,N2 (cm3¨ g´1) - 0.35 0.44 0.32 0.50L0,N2 (nm) - 1.80 1.00 1.23 0.82W0,CO2 (cm3¨ g´1) 0.21 0.18 0.38 0.31 0.27L0,CO2 (nm) 0.58 0.70 0.71 0.64 0.64

As expected, in agreement with the shape of the corresponding isotherms, most of the porosity inNKa-A82, NKb-A82 and NOS-A94 lies in the microporosity domain. To be more precise, NKb-A82concentrates its microporosity in the narrow microporosity range, whereas NKa-A82 and, particularly,NOS-A94 have a wider microporosity. Samples RKa-A71 and RKb-A75 show similar narrow microporevolumes (W0,CO2). The textural development of RKa-A71 is restricted to this narrow microporosity,whereas RKb-A75 also presents supermicropores (1–2 nm) and mesopores (2–50 nm) as can be inferredfrom the differences between Vp, W0,N2 and W0,CO2. Average micropore widths (L0,N2) of around 1 nmfor the NK and NOS based ACs and ca. 2 nm for the RKb-A75 sample were assessed. The narrowmicroporosity (L0,CO2) fell within the 0.6–0.7 nm range. The route selected to synthesise the PFprecursor compromised the textural development that could have been achieved for the activatedcarbon. In order to gain a better insight into the narrow microporosity, the pore size distributionsin the range of 0.3–1 nm were estimated by means of the Density Functional Theory (DFT) for CO2

adsorption onto slit pore carbons using Micromeritics’ software (method: non-negative regularization,little smoothing). The plots are shown in Figure 2.Energies 2016, 9, 189 7 of 18

Figure 2. Micropore size distributions from the CO2 adsorption isotherms at 0 °C by means of the

DFT slit pore carbon model.

The narrow micropore size distributions of the studied materials can be described as bimodal as

they present two main peaks with characteristic pore sizes in the range of 0.55–0.65 and 0.8–0.9 nm,

respectively. The bimodal shape of the pore size distributions is a characteristic feature for many

adsorbents possessing even a small amount of micropores. It is shown that this feature results from

the similarity of the local adsorption isotherm in the range of the pore widths for which the gap

between peaks (related to the primary and secondary micropore filling mechanism) exists [25].

Generally, the tested carbons show striking similarities in the pore size distributions. Sample

RKa-A71 seems the exception as it shows a narrower micropore size distribution. The sizes of the

narrow micropores in the materials are highly suitable for the separation of CO2 at sub-atmospheric

pressures [15,20].

3.2. CO2 Uptake Screening

The CO2 adsorption capacity of the produced carbons was first evaluated at sub-atmospheric

pressures and room temperature. CO2 adsorption isotherms at 25 °C and up to 101 kPa were

determined in a Quantachrome Nova 4000 volumetric device. The samples were previously

outgassed overnight at 100 °C under vacuum. A commercial activated carbon, Calgon BPL, was also

tested for reference purposes [26]. This is a Calgon Carbon granular activated carbon specifically

designed for use in gas phase applications. The measured isotherms are plotted in Figure 3.

-0.05

0.15

0.35

0.55

0.75

0.95

1.15

1.35

1.55

1.75

0.30 0.50 0.70 0.90

Dif

fere

nti

al p

ore

vo

lum

e (

cm3

g-1)

Pore width (nm)

RKa-A71

RKb-A75

NKa-A82

NKb-A82

NOS-A94

Figure 2. Micropore size distributions from the CO2 adsorption isotherms at 0 ˝C by means of the DFTslit pore carbon model.

The narrow micropore size distributions of the studied materials can be described as bimodal asthey present two main peaks with characteristic pore sizes in the range of 0.55–0.65 and 0.8–0.9 nm,

Energies 2016, 9, 189 7 of 17

respectively. The bimodal shape of the pore size distributions is a characteristic feature for manyadsorbents possessing even a small amount of micropores. It is shown that this feature results from thesimilarity of the local adsorption isotherm in the range of the pore widths for which the gap betweenpeaks (related to the primary and secondary micropore filling mechanism) exists [25].

Generally, the tested carbons show striking similarities in the pore size distributions. SampleRKa-A71 seems the exception as it shows a narrower micropore size distribution. The sizes of thenarrow micropores in the materials are highly suitable for the separation of CO2 at sub-atmosphericpressures [15,20].

3.2. CO2 Uptake Screening

The CO2 adsorption capacity of the produced carbons was first evaluated at sub-atmosphericpressures and room temperature. CO2 adsorption isotherms at 25 ˝C and up to 101 kPa weredetermined in a Quantachrome Nova 4000 volumetric device. The samples were previously outgassedovernight at 100 ˝C under vacuum. A commercial activated carbon, Calgon BPL, was also tested forreference purposes [26]. This is a Calgon Carbon granular activated carbon specifically designed foruse in gas phase applications. The measured isotherms are plotted in Figure 3.

Energies 2016, 9, 189 8 of 18

Figure 3. CO2 adsorption isotherms at 25 °C and sub-atmospheric pressures for the studied carbons.

The NK and NOS carbons showed substantial CO2 uptakes over the sub-atmospheric pressure

range tested, with isotherms characterised by more pronounced slopes at pressures below 20 kPa.

Maximum CO2 uptakes of 2.9, 2.4 and 2.3 mol·kg−1 were achieved at 101 kPa for samples NKa-A82,

NKb-A82 and NOS-A94, respectively. The isotherms of the two NK samples nearly overlapped in

the pressure range below 20 kPa, as might be expected from the similarities in the narrower

microporosity of both samples. When the pressure approaches the atmospheric value, differences in

the CO2 adsorption capacity appeared revealing the different microporosity features of the two NK

activated carbons. The performance of carbon NOS shows striking similarities to that of NKb in

agreement with the similarities in the narrow microporosity of both samples. The RK carbons

showed similar performances to that of the commercial activated carbon over the tested pressure

range with maximum CO2 uptakes of less than 2 mol·kg−1 at 101 kPa. Taking 2 mol·kg−1 as the

screening value for selecting optimum CO2 adsorbents, RK carbons were discarded for further study.

3.3. Binary Adsorption from Mixtures of CO2-H2, CO2-CH4, and CO2-N2

3.3.1. Single-Component Adsorption Isotherms

H2, CH4, and N2 adsorption isotherms at 25 °C and sub-atmospheric pressures (up to 100.3 kPa)

for the NK and NOS carbons were obtained using a Micromeritics TriStar 3000 volumetric

equipment. BPL was also tested for comparison purposes. Detailed textural characteristics of this

activated carbon can be found elsewhere [27]. Prior to analysis the samples were outgassed

overnight at 100 °C under vacuum.

Experimental data of the single component CO2, H2, CH4, and N2 adsorption isotherms were

fitted to the Sips equation, as follows:

Sips equation:

n

n

sbP

bPqq

/1

/1

1 (1)

where q represents the concentration of the adsorbed species, qs the saturation capacity and P the

pressure of the adsorptive. The parameters b, n and qs are temperature dependent; b usually takes the

form of the adsorption affinity. The parameter n shows the heterogeneity of the system and its value

is usually greater than unity; therefore, the larger the value of n, the more heterogeneous the system is.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 20 40 60 80 100

qC

O2

(mo

l kg-1

)

Pressure (kPa)

RKa-A71

RKb-A75

NKa-A82

NKb-A82

NOS-A94

BPL

Figure 3. CO2 adsorption isotherms at 25 ˝C and sub-atmospheric pressures for the studied carbons.

The NK and NOS carbons showed substantial CO2 uptakes over the sub-atmospheric pressurerange tested, with isotherms characterised by more pronounced slopes at pressures below 20 kPa.Maximum CO2 uptakes of 2.9, 2.4 and 2.3 mol¨kg´1 were achieved at 101 kPa for samples NKa-A82,NKb-A82 and NOS-A94, respectively. The isotherms of the two NK samples nearly overlapped in thepressure range below 20 kPa, as might be expected from the similarities in the narrower microporosityof both samples. When the pressure approaches the atmospheric value, differences in the CO2

adsorption capacity appeared revealing the different microporosity features of the two NK activatedcarbons. The performance of carbon NOS shows striking similarities to that of NKb in agreementwith the similarities in the narrow microporosity of both samples. The RK carbons showed similarperformances to that of the commercial activated carbon over the tested pressure range with maximumCO2 uptakes of less than 2 mol¨ kg´1 at 101 kPa. Taking 2 mol¨ kg´1 as the screening value for selectingoptimum CO2 adsorbents, RK carbons were discarded for further study.

Energies 2016, 9, 189 8 of 17

3.3. Binary Adsorption from Mixtures of CO2-H2, CO2-CH4, and CO2-N2

3.3.1. Single-Component Adsorption Isotherms

H2, CH4, and N2 adsorption isotherms at 25 ˝C and sub-atmospheric pressures (up to 100.3 kPa)for the NK and NOS carbons were obtained using a Micromeritics TriStar 3000 volumetric equipment.BPL was also tested for comparison purposes. Detailed textural characteristics of this activatedcarbon can be found elsewhere [27]. Prior to analysis the samples were outgassed overnight at 100 ˝Cunder vacuum.

Experimental data of the single component CO2, H2, CH4, and N2 adsorption isotherms werefitted to the Sips equation, as follows:

Sips equation: q “ qspbPq1{n

1` pbPq1{n(1)

where q represents the concentration of the adsorbed species, qs the saturation capacity and P thepressure of the adsorptive. The parameters b, n and qs are temperature dependent; b usually takes theform of the adsorption affinity. The parameter n shows the heterogeneity of the system and its value isusually greater than unity; therefore, the larger the value of n, the more heterogeneous the system is.

This empirical three-parameter equation has been widely used for fitting the isotherm data ofdifferent hydrocarbons on activated carbon. The Sips equation is believed to give a more accuratefit over a larger pressure regime than the standard Langmuir or Freundlich equations, and providesmore accurate prediction of the quantity adsorbed at saturation than the Langmuir equation forheterogeneous adsorbents. However, it suffers from the same disadvantage as the Freundlich equation:it does not reduce to Henry’s Law at low surface coverage [28].

Fitting of the model to the experimental data was conducted by means of the Solver Excel tool(Microsoft Office Excel 2010) departing from values of qs and n of 1 and b of 0. The goodness ofthe fit was evaluated on the basis of the minimum squared relative error (SRE) as given by thefollowing expression:

SREp%q “

g

f

f

e

ř

i

“`

qexp,i ´ qmod,i˘

{qexp,i‰2

N ´ 1ˆ 100 (2)

where qexp,i and qmod,i are the experimental and Sips-predicted adsorbed amounts of component i,respectively, and N is the number of experimental data points. The optimal parameters and squaredrelative errors from fitting the Sips model to the single-component data of CO2, H2, CH4, and N2 at25 ˝C are summarized in Table 5. Isotherm experimental data and fittings to the Sips model are shownin Figure 4.

The experimental adsorption data show that much more CO2 is adsorbed than CH4 and moresignificantly than N2 and H2 on the tested carbons. The differences in CO2, CH4 and N2 uptake maybe explained by the larger quadrupole moment of CO2 that produces a strong attraction towardsthe adsorbent surface resulting in a greater uptake. The high level of polarizability of CO2 andCH4 can create a momentary shift in its neutral electrostatic field but this attraction force is muchweaker than the quadrupole moment [29]. In contrast, N2 molecules are nonpolar and smaller,compared with CH4 molecules. For this reason, the differences in CO2 and N2 uptake are greaterthan the differences in CO2 and CH4 uptake. Since the narrow micropores allow the strongestinteraction between the carbonaceous solid and the hydrogen molecule, it is to be expected thatthis fraction of the porous volume will make a major contribution to the hydrogen adsorptioncapacity of the materials. Although hydrogen should only interact through induced polarization,surface chemistry may influence hydrogen adsorption on carbonaceous surfaces, particularly atsub-atmospheric pressures [30,31].

Energies 2016, 9, 189 9 of 17

Table 5. Optimal parameters of the single-component CO2, CH4, H2, and N2 adsorption data fitting tothe Sips model.

SampleCO2

SRE (%)CH4

SRE (%)qs b n qs b n

NKa-A82 7.37 5.77 ˆ 10´3 1.17 1.1 3.76 5.69 ˆ 10´3 1.10 1.5NKb-A82 5.38 7.46 ˆ 10´3 1.24 1.0 3.37 3.13 ˆ 10´3 1.15 1.9NOS-A94 6.29 4.77 ˆ 10´3 1.28 0.6 4.20 2.62 ˆ 10´3 1.30 3.3BPL 4.72 6.02 ˆ 10´3 1.17 3.0 2.63 4.03 ˆ 10´3 1.07 2.6

SampleH2

SRE (%)N2

SRE (%)qs b n qs b n

NKa-A82 0.39 1.50 ˆ 10´3 1.00 8.8 2.18 2.55 ˆ 10´3 1.00 1.7NKb-A82 2.46 1.29 ˆ 10´4 1.00 8.2 2.87 1.17 ˆ 10´3 1.00 2.1NOS-A94 1.44 4.06 ˆ 10´5 1.65 11.0 2.36 1.81 ˆ 10´3 1.00 2.1BPL 0.41 1.98 ˆ 10´4 1.19 5.1 2.90 9.89 ˆ 10´4 1.00 2.0

qs: mol¨kg´1, b: kPa´1.Energies 2016, 9, 189 10 of 18

(a) (b)

(c)

Figure 4. CO2, CH4 (a); H2 (b) and N2 (c) adsorption isotherms at 25 °C on samples NKa-A82,

NKb-A82, NOS-A94 and BPL. Fitting of the single component adsorption data to the Sips model is

represented by the solid lines.

By comparing the quantities adsorbed by the phenol-formaldehyde carbons in Figure 4 it can be

seen that NKa-A82 is the sample with the greatest CO2, CH4, N2 and H2 uptakes over the

sub-atmospheric pressure range tested. Moreover, the adsorption capacities of the three PF carbons

are greater than that of the reference carbon, BPL. In terms of CO2 adsorption, the performance at

pressures below 20 kPa is quite similar for the NK and NOS carbons and it is only at higher

pressures where NKa-A82 stands out. CH4 adsorption isotherms show differentiated uptakes for the

three PF carbons that follow the order NKa > NOS > NKb. All three samples present relevant

microporosity; in sample NKb narrow micropores (<1 nm) are mainly encountered, whereas for NKa

and NOS microporosity >1 nm is also present, in agreement with the textural parameters estimated

from the DR relation (see Table 4). In this scenario selectivity effects related to the kinetic diameters

of the two gas molecules (3.30 Å for CO2 and 3.80 Å for CH4) [29] may play a role in the change in the

CO2 and CH4 adsorption trends of the three PF carbons. N2 and H2 adsorption is characterised by

linear isotherms that reveal the unspecific nature of the adsorbate-adsorbent interaction [32].

The pure component CO2, H2, CH4, and N2 adsorption isotherms fitted to the Sips model

(Equation (1)) are represented by the continuous lines in Figure 4. The saturation capacities (qs) and

the parameter n have been fitted to the adsorption data for the four gas species independently (Table 5).

The Sips model reproduces the adsorption of CO2, CH4, and N2 with good accuracy, with

substantial deviations for the adsorption of H2. It should be noted that the experimental H2

adsorption data at sub-atmospheric pressures and ambient temperature are subject to greater

uncertainty due to the significantly lower uptakes of H2 under these conditions. The values of qs and

b follow the trend CO2 > CH4 > N2 >> H2, as might be expected from the experimental data where the

capacity and, consequently, the affinity of the carbons to adsorb CO2 and CH4 is one and two orders

of magnitude greater than that of N2 and H2, respectively. The parameter n takes values close to

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 20 40 60 80 100

q (

mo

l kg-1

)

Pressure (kPa)

NKa-A82NKb-A82NOS-A94BPLSips fitting

CO2

CH4

0.0

0.2

0.4

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 20 40 60 80 100

qH

2(m

ol k

g-1)

qC

O2

(mo

l kg-1

)

Pressure (kPa)

CO2

H2

0

0.5

1

1.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 20 40 60 80 100

qN

2(m

ol k

g-1)

qC

O2

(mo

l kg-1

)

Pressure (kPa)

CO2

N2

Figure 4. CO2, CH4 (a); H2 (b) and N2 (c) adsorption isotherms at 25 ˝C on samples NKa-A82,NKb-A82, NOS-A94 and BPL. Fitting of the single component adsorption data to the Sips model isrepresented by the solid lines.

By comparing the quantities adsorbed by the phenol-formaldehyde carbons in Figure 4 it canbe seen that NKa-A82 is the sample with the greatest CO2, CH4, N2 and H2 uptakes over thesub-atmospheric pressure range tested. Moreover, the adsorption capacities of the three PF carbonsare greater than that of the reference carbon, BPL. In terms of CO2 adsorption, the performance atpressures below 20 kPa is quite similar for the NK and NOS carbons and it is only at higher pressureswhere NKa-A82 stands out. CH4 adsorption isotherms show differentiated uptakes for the three PFcarbons that follow the order NKa > NOS > NKb. All three samples present relevant microporosity;in sample NKb narrow micropores (<1 nm) are mainly encountered, whereas for NKa and NOSmicroporosity >1 nm is also present, in agreement with the textural parameters estimated from the DR

Energies 2016, 9, 189 10 of 17

relation (see Table 4). In this scenario selectivity effects related to the kinetic diameters of the two gasmolecules (3.30 Å for CO2 and 3.80 Å for CH4) [29] may play a role in the change in the CO2 and CH4

adsorption trends of the three PF carbons. N2 and H2 adsorption is characterised by linear isothermsthat reveal the unspecific nature of the adsorbate-adsorbent interaction [32].

The pure component CO2, H2, CH4, and N2 adsorption isotherms fitted to the Sips model(Equation (1)) are represented by the continuous lines in Figure 4. The saturation capacities (qs) and theparameter n have been fitted to the adsorption data for the four gas species independently (Table 5).

The Sips model reproduces the adsorption of CO2, CH4, and N2 with good accuracy, withsubstantial deviations for the adsorption of H2. It should be noted that the experimental H2 adsorptiondata at sub-atmospheric pressures and ambient temperature are subject to greater uncertainty due tothe significantly lower uptakes of H2 under these conditions. The values of qs and b follow the trendCO2 > CH4 > N2 >> H2, as might be expected from the experimental data where the capacity and,consequently, the affinity of the carbons to adsorb CO2 and CH4 is one and two orders of magnitudegreater than that of N2 and H2, respectively. The parameter n takes values close to unity for CO2

and CH4 while for N2 takes values of unity, suggesting that heterogeneity plays a minor role in thesesystems. The uneven fitting of the H2 isotherms for the studied carbons can be mainly ascribed touncertainty of the experimental data. Sample NOS is where greatest deviation from unity in the valuesof n is found for the adsorption of CO2, CH4 and H2.

3.3.2. Multicomponent Adsorption Prediction

In order to predict multicomponent adsorption from binary CO2-H2, CO2-CH4, and CO2-N2

mixtures, the fitting of the single component adsorption data to the Sips model represented by Equation(1) was rerun to account for the interaction between components in each binary mixture. Given thatCO2 is the strongest adsorbate, the fitting was conducted keeping qs constant (the value of qs estimatedfrom the fitting of single component CO2 adsorption isotherm for each sample) and adjusting the band n values to minimize the relative error for the pairs CO2-H2, CO2-CH4, and CO2-N2, respectively.The values of the parameters estimated from these fittings are collected in Table 6.

Table 6. Optimal parameters of the single-component CO2, H2, CH4, and N2 adsorption data fitting tothe Sips model for multicomponent prediction.

SampleCO2-CH4

bCO2 bCH4 n SRE (%)

NKa-A82 5.83 ˆ 10´3 2.01 ˆ 10´3 1.16 3.0NKb-A82 7.99 ˆ 10´3 1.52 ˆ 10´3 1.19 2.7NOS-A94 4.43 ˆ 10´3 1.36 ˆ 10´3 1.32 2.3BPL 6.33 ˆ 10´3 1.60 ˆ 10´3 1.14 3.7

SampleCO2-H2

bCO2 bH2 n SRE (%)

NKa-A82 6.55 ˆ 10´3 4.27 ˆ 10´5 1.10 7.9NKb-A82 1.03 ˆ 10´2 5.33 ˆ 10´5 1.01 15.4NOS-A94 4.18 ˆ 10´3 1.69 ˆ 10´5 1.35 9.6BPL 6.08 ˆ 10´3 1.31 ˆ 10´5 1.16 4.3

SampleCO2-N2

bCO2 bN2 n SRE (%)

NKa-A82 6.93 ˆ 10´3 5.36 ˆ 10´4 1.06 6.6NKb-A82 9.63 ˆ 10´3 6.98 ˆ 10´4 1.06 9.6NOS-A94 6.90 ˆ 10´3 5.12 ˆ 10´4 1.05 11.9BPL 7.00 ˆ 10´3 3.75 ˆ 10´4 1.10 10.7

b: kPa´1.

The fitted parameters follow the same trend observed in the previous independent Sips fittings:the n values are close to unity and the b values follow the order CO2 > CH4 > N2 >> H2. When mixedthe affinity, as represented by the parameter b, increases for the strong adsorbate (CO2) when compared

Energies 2016, 9, 189 11 of 17

to the pure component CO2 adsorption data independent fitting. Likewise, the weaker adsorbateshows a substantial decrease in affinity particularly in the CO2-H2 and CO2-N2 systems.

In this way, use of the parameters from the fitting of pure component data pairs guaranteesthe thermodynamic consistency of the resulting analytical expression of the multicomponent Sipsmodel. Using the same analogy for extending the single-component Langmuir equation to that formulticomponent adsorption, the following equation is obtained for the multicomponent Sips model [33]:

qi “qs pbiyiPq

1{n

1`Nř

j“1

`

bjyjP˘1{n

(3)

where i and j stand for the components in the binary mixture, y is the mole fraction of the correspondingcomponent in the gas phase, and N is the number of components in the gas mixture, i.e., 2. However,this equation only corresponds to a specific case of surface energetic heterogeneity. For normal activatedcarbons in which the energy sites are highly correlated, the Ideal Adsorbed Solution Theory (IAST) shouldbe used. Application of the IAST with the concept of hypothetical pure-component pressure results in thefollowing equation [34]:

qi “

qsbiyiP

˜

Nř

j“1bjyjP

¸p1{nq´1

1`

˜

Nř

j“1bjyjP

¸1{n(4)

It can be seen that Equations (3) and (4) deliver different expressions to reproduce the multicomponentSips model on the basis of different assumptions regarding the heterogeneity of the adsorbate-adsorbentsystem. Nevertheless, both equations were used in the present study to predict the binary adsorptionequilibrium for CO2-H2, CO2-CH4, and CO2-N2 systems and the results are discussed and compared.The parameter values used for such multicomponent adsorption predictions were those previously obtainedfrom the combined fitting of the pure component adsorption data of the pairs of gases in each mixture asshown in Table 6.

The predictions of the binary gas adsorption equilibria from the multicomponent Sips modelrepresented by Equations (3) and (4) and from the parameters included in Table 6 are depicted inFigures 5–7. Multicomponent predictions were assessed under the following temperature and pressureconditions: 25 ˝C and 100 kPa.

Energies 2016, 9, 189 12 of 18

N

j

n

jj

n

iisi

Pyb

Pybqq

1

/1

/1

1

(3)

where i and j stand for the components in the binary mixture, y is the mole fraction of the

corresponding component in the gas phase, and N is the number of components in the gas mixture,

i.e., 2. However, this equation only corresponds to a specific case of surface energetic heterogeneity.

For normal activated carbons in which the energy sites are highly correlated, the Ideal Adsorbed

Solution Theory (IAST) should be used. Application of the IAST with the concept of hypothetical

pure-component pressure results in the following equation [34]:

nN

j

jj

nN

j

jjiis

i

Pyb

PybPybq

q/1

1

1)/1(

1

1

(4)

It can be seen that Equations (3) and (4) deliver different expressions to reproduce the

multicomponent Sips model on the basis of different assumptions regarding the heterogeneity of the

adsorbate-adsorbent system. Nevertheless, both equations were used in the present study to predict

the binary adsorption equilibrium for CO2-H2, CO2-CH4, and CO2-N2 systems and the results are

discussed and compared. The parameter values used for such multicomponent adsorption

predictions were those previously obtained from the combined fitting of the pure component

adsorption data of the pairs of gases in each mixture as shown in Table 6.

The predictions of the binary gas adsorption equilibria from the multicomponent Sips model

represented by Equations (3) and (4) and from the parameters included in Table 6 are depicted in

Figures 5–7. Multicomponent predictions were assessed under the following temperature and

pressure conditions: 25 °C and 100 kPa.

Globally, the predicted multicomponent adsorption isotherms show a reduction in the uptakes

of the four components when mixed in binary CO2-H2, CO2-CH4 or CO2-N2 mixtures. However,

while the CO2 uptake when mixed in a 1:1 molar ratio is reduced to 60%, 70% and 75% (average

values for the four evaluated carbons in the CO2-CH4, CO2-N2 and CO2-H2 binary mixtures,

respectively) of the pure gas CO2 adsorption at 25 °C and 100 kPa, the H2, N2 and CH4 uptakes

drastically drop down to around 30–35% of those corresponding to pure gas adsorption. This

suggests that the presence of another component in the mixture influences the adsorption of CO2 to a

lesser extent.

0.00

0.02

0.04

0.06

0.08

0.10

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 0.2 0.4 0.6 0.8 1

qH

2(m

ol k

g-1)

qC

O2

(m

ol k

g-1)

yH2

a)

0.00

0.02

0.04

0.06

0.08

0.10

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 0.2 0.4 0.6 0.8 1

qH

2(m

ol k

g-1)

qC

O2

(m

ol k

g-1)

yH2

CO2-Sips eq 3

CO2-Sips eq 4

H2- Sips eq 3

H2-Sips eq 4

b)

Figure 5. Cont.

Energies 2016, 9, 189 12 of 17

Energies 2016, 9, 189 13 of 18

Figure 5. Multicomponent prediction by means of the Sips model for CO2-H2 binary mixtures at 100

kPa and 25 °C: (a) NKa-A82; (b) NKb-A82; (c) NOS-A94 and (d) BPL.

Figure 6. Multicomponent prediction by means of the Sips model for CO2-CH4 binary mixtures at 100

kPa and 25 °C: (a) NKa-A82; (b) NKb-A82; (c) NOS-A94 and (d) BPL.

0.00

0.02

0.04

0.06

0.08

0.10

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 0.2 0.4 0.6 0.8 1

qH

2(m

ol k

g-1)

qC

O2

(m

ol k

g-1)

yH2

c)

0.00

0.02

0.04

0.06

0.08

0.10

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 0.2 0.4 0.6 0.8 1

qH

2(m

ol k

g-1)

qC

O2

(m

ol k

g-1)

yH2

d)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 0.2 0.4 0.6 0.8 1

q (

mo

l kg-1

)

yCH4

a)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 0.2 0.4 0.6 0.8 1

q (

mo

l kg-1

)

yCH4

CO2-Sips eq 3

CO2-Sips eq 4

CH4- Sips eq 3

CH4-Sips eq 4

b)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 0.2 0.4 0.6 0.8 1

q (

mo

l kg-1

)

yCH4

c)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 0.2 0.4 0.6 0.8 1

q (

mo

l kg-1

)

yCH4

d)

Figure 5. Multicomponent prediction by means of the Sips model for CO2-H2 binary mixtures at 100kPa and 25 ˝C: (a) NKa-A82; (b) NKb-A82; (c) NOS-A94 and (d) BPL.

Energies 2016, 9, 189 13 of 18

Figure 5. Multicomponent prediction by means of the Sips model for CO2-H2 binary mixtures at 100

kPa and 25 °C: (a) NKa-A82; (b) NKb-A82; (c) NOS-A94 and (d) BPL.

Figure 6. Multicomponent prediction by means of the Sips model for CO2-CH4 binary mixtures at 100

kPa and 25 °C: (a) NKa-A82; (b) NKb-A82; (c) NOS-A94 and (d) BPL.

0.00

0.02

0.04

0.06

0.08

0.10

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 0.2 0.4 0.6 0.8 1

qH

2(m

ol k

g-1)

qC

O2

(m

ol k

g-1)

yH2

c)

0.00

0.02

0.04

0.06

0.08

0.10

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 0.2 0.4 0.6 0.8 1

qH

2(m

ol k

g-1)

qC

O2

(m

ol k

g-1)

yH2

d)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 0.2 0.4 0.6 0.8 1

q (

mo

l kg-1

)

yCH4

a)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 0.2 0.4 0.6 0.8 1

q (

mo

l kg-1

)

yCH4

CO2-Sips eq 3

CO2-Sips eq 4

CH4- Sips eq 3

CH4-Sips eq 4

b)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 0.2 0.4 0.6 0.8 1

q (

mo

l kg-1

)

yCH4

c)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 0.2 0.4 0.6 0.8 1

q (

mo

l kg-1

)

yCH4

d)

Figure 6. Multicomponent prediction by means of the Sips model for CO2-CH4 binary mixtures at100 kPa and 25 ˝C: (a) NKa-A82; (b) NKb-A82; (c) NOS-A94 and (d) BPL.

Energies 2016, 9, 189 13 of 17Energies 2016, 9, 189 14 of 18

Figure 7. Multicomponent prediction by means of the Sips model for CO2-N2 binary mixtures at 100

kPa and 25 °C: (a) NKa-A82; (b) NKb-A82; (c) NOS-A94 and (d) BPL.

It is apparent that when n (Table 6) approaches unity the values of the numerator and the

denominator of Equations (3) and (4) tend to match each other and both Equations (3) and (4) give

similar predictions. This suggests that there is little heterogeneity in the systems that could influence

the adsorption of the binary mixtures onto the evaluated carbons. The CO2-H2 system is where

greater deviations between the two predictions are observed, particularly for the H2 uptake. It

should be noted that at sub-atmospheric pressures and room temperature experimental H2

adsorption is characterised by extremely low uptakes. In addition, the affinity of the carbons

towards H2 adsorption is significantly lower than that to CO2. Consequently, binary adsorption

prediction for the CO2-H2 system is subject to greater uncertainty. Moreover, predicted CO2

adsorption in the CO2-H2 binary mixtures nearly matches pure component CO2 adsorption at similar

partial pressures suggesting that H2 adsorption in these mixtures can be considered as negligible.

This is in agreement with previous results from the research team [6,35].

The CO2 uptake at yj = 0 (where j stands for H2, N2 and CH4, respectively) is slightly

overestimated for the CO2-H2 and CO2-N2 systems compared to the pure component CO2 adsorption

data. This may be attributed to the different affinity of the adsorbent towards CO2 and H2 or N2.

In the case of CO2-CH4 (Figure 6) and CO2-N2 (Figure 7) the predictions of the two Equations (3)

and (4) seem consistent. The values of n (Table 6) are close to unity for the CO2-N2 system and thus

Equations 3 and 4 deliver similar predictions. For the phenolic resin-biomass composite derived

carbon, NOS-A94, Equation (3) predicts slightly greater CH4 uptakes (+0.15 mol·kg−1) than Equation

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 0.2 0.4 0.6 0.8 1

q(m

ol k

g-1)

yN2

a)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 0.2 0.4 0.6 0.8 1

q(m

ol k

g-1)

yN2

CO2-Sips eq 3CO2-Sips eq 4N2- Sips eq 3N2-Sips eq 4

b)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 0.2 0.4 0.6 0.8 1

q(m

ol k

g-1)

yN2

c)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 0.2 0.4 0.6 0.8 1

q(m

ol k

g-1)

yN2

d)

Figure 7. Multicomponent prediction by means of the Sips model for CO2-N2 binary mixtures at 100 kPaand 25 ˝C: (a) NKa-A82; (b) NKb-A82; (c) NOS-A94 and (d) BPL.

Globally, the predicted multicomponent adsorption isotherms show a reduction in the uptakes ofthe four components when mixed in binary CO2-H2, CO2-CH4 or CO2-N2 mixtures. However, whilethe CO2 uptake when mixed in a 1:1 molar ratio is reduced to 60%, 70% and 75% (average values forthe four evaluated carbons in the CO2-CH4, CO2-N2 and CO2-H2 binary mixtures, respectively) of thepure gas CO2 adsorption at 25 ˝C and 100 kPa, the H2, N2 and CH4 uptakes drastically drop downto around 30–35% of those corresponding to pure gas adsorption. This suggests that the presence ofanother component in the mixture influences the adsorption of CO2 to a lesser extent.

It is apparent that when n (Table 6) approaches unity the values of the numerator and thedenominator of Equations (3) and (4) tend to match each other and both Equations (3) and (4) givesimilar predictions. This suggests that there is little heterogeneity in the systems that could influencethe adsorption of the binary mixtures onto the evaluated carbons. The CO2-H2 system is where greaterdeviations between the two predictions are observed, particularly for the H2 uptake. It should be notedthat at sub-atmospheric pressures and room temperature experimental H2 adsorption is characterisedby extremely low uptakes. In addition, the affinity of the carbons towards H2 adsorption is significantlylower than that to CO2. Consequently, binary adsorption prediction for the CO2-H2 system is subjectto greater uncertainty. Moreover, predicted CO2 adsorption in the CO2-H2 binary mixtures nearlymatches pure component CO2 adsorption at similar partial pressures suggesting that H2 adsorption in

Energies 2016, 9, 189 14 of 17

these mixtures can be considered as negligible. This is in agreement with previous results from theresearch team [6,35].

The CO2 uptake at yj = 0 (where j stands for H2, N2 and CH4, respectively) is slightly overestimatedfor the CO2-H2 and CO2-N2 systems compared to the pure component CO2 adsorption data. This maybe attributed to the different affinity of the adsorbent towards CO2 and H2 or N2.

In the case of CO2-CH4 (Figure 6) and CO2-N2 (Figure 7) the predictions of the twoEquations (3) and (4) seem consistent. The values of n (Table 6) are close to unity for the CO2-N2 systemand thus Equations 3 and 4 deliver similar predictions. For the phenolic resin-biomass compositederived carbon, NOS-A94, Equation (3) predicts slightly greater CH4 uptakes (+0.15 mol¨kg´1) thanEquation (4) for molar fractions of methane (yCH4) in the binary mixture between 0.3 and 0.6. Both CO2

and CH4 show specific interaction with the carbon surface, as reflected by the shape of the isotherms(type I, according to the IUPAC classification). However, despite the preferential adsorption of CO2

over CH4, competition between both adsorbates exists. The significantly higher critical temperature ofCO2 in comparison with CH4 makes carbon dioxide more likely to behave as a condensable steam thanas a supercritical gas, making it less volatile and increasing its chances of adsorption. Moreover, CO2

presents a higher polarizability which may enhance attractive forces with the surface and a permanentquadrupole, leading to stronger interactions with any solid surface.

The simplified extended Sips model (Equation (3)) shows limitations bearing in mind theassumptions needed to extend the single component Sips equation to the multicomponentone. However, the observed deviations between the multicomponent predictions represented byEquations (3) and (4) are small and therefore, both could be applied to predict the adsorption ofCO2, CH4, and N2 on the evaluated carbons in binary CO2-CH4, and CO2-N2 mixtures. The H2

uptakes predicted for the CO2-H2 systems should be handled with care given the uncertaintyof the experimental H2 adsorption data under sub-atmospheric pressures and room temperature.H2 adsorption can be regarded as negligible in the CO2-H2 systems. Moreover, data on the performanceof the phenolic resin-derived carbon for CO2-CH4 adsorption were also experimentally obtained in alab scale fixed-bed adsorption unit. Details can be found elsewhere [6]. The CO2 and CH4 uptakesassessed at 120 kPa and 25 ˝C for a 1:1 CO2:CH4 binary mixture are in good agreement with themulticomponent predictions at 100 kPa and 25 ˝C presented herein; for instance, the predicted CO2

uptake on carbon NKa-A82 for a 1:1 CO2:CH4 mixture at 120 kPa is 1.90 mol¨ kg´1 in agreement with2.03 mol¨kg´1 estimated experimentally.

3.4. Selectivity to CO2 Separation

The selectivity of the prepared carbons to separate CO2 from CO2-H2, CO2-N2 and CO2-CH4

mixtures at ambient temperature and sub-atmospheric pressures was evaluated on the basis of themulticomponent adsorption predictions. The following expression was used:

S1{2 “q1{q2

y1{y2(5)

where q1 stands for the CO2 uptake of the carbon from the binary mixture and q2 represents theH2, N2 and CH4 uptakes, respectively. y1 and y2 are the corresponding molar fractions of the gascomponent in the binary mixtures. From the IAST-Sips multicomponent adsorption model representedby Equation (4), the following equation can be inferred to account for the selectivity to separate CO2

from binary mixtures:

S1{2 “b1

b2(6)

where b1 and b2 stand for the affinity constants of CO2 and H2, N2 and CH4, respectively. Accordingto Equation (6) the selectivity to separate CO2 from binary mixtures is independent of the partialpressures of the components in the binary mixture. Table 7 summarizes the selectivities of the carbonsas estimated from Equation (6) (b1:bCO2 and b2:bH2, bN2 or bCH4).

Energies 2016, 9, 189 15 of 17

Table 7. Selectivities of the carbons to separate CO2 from CO2-H2, CO2-N2 and CO2-CH4

binary mixtures.

S1/2 CO2-H2 CO2-N2 CO2-CH4

NKa-A82 153 12.9 2.9NKb-A82 193 13.8 5.3NOS-A94 247 13.5 3.3BPL 464 18.7 3.9

The selectivity values follow the order CO2-H2 >> CO2-N2 > CO2-CH4 indicating that theseparation of CO2 from CO2-H2 mixtures at sub-atmospheric pressures and ambient temperature isfavoured. BPL shows the greatest selectivity values to separate CO2 from CO2-H2 and CO2-N2 binarymixtures whereas the phenolic resin-derived carbon NKb-A82 presents the highest selectivity forCO2 to be separated from CO2-CH4 mixtures. Nevertheless, selectivity values for the four evaluatedcarbons are in the same order of magnitude for each mixture and the values are particularly close inthe case of CO2-N2 and CO2-CH4 separation.

The selectivity value, as estimated from Equation (6), could serve as a rough sorbent selectionparameter. However, for a more precise evaluation the working capacities (differences between the gasuptakes of both components under adsorption and regeneration conditions) and the heat required forthe regeneration of the adsorbent would also need to be taken into account [36].

4. Conclusions

Phenolic resin-derived carbons were prepared and evaluated for CO2 adsorption atsub-atmospheric pressures. Single component adsorption isotherms of CO2, H2, N2 and CH4 weremeasured at 25 ˝C and up to 100 kPa and fitted to the Sips model. Binary adsorption from CO2-H2,CO2-N2 and CO2-CH4 mixtures was then predicted by applying the extended multicomponentSips model and the Ideal Adsorbed Solution Theory (IAST) in conjunction with the Sips model.Both equations showed similar predictions for the adsorption of CO2, CH4 and N2 on the evaluatedcarbons in the binary mixtures. The adsorption of H2 at the conditions evaluated can be considerednegligible. Both Sips multicomponent equations might predict binary adsorption in the studiedsystems. This is a significant advantage from the viewpoint of computational modelling requirementsin adsorption process design.

In terms of CO2 uptake, the performance of the acid resin-derived carbons in separating CO2

from CO2-CH4, CO2-H2, and CO2-N2 at atmospheric pressure was superior to that of a referencecommercial carbon (Calgon BPL) and an even higher selectivity towards CO2 was found in theseparation of CO2-CH4.

Acknowledgments: This work was carried out with financial support from the Spanish MINECO (ProjectENE2011-23467), and was co-financed by the European Regional Development Fund (ERDF) and the Gobiernodel Principado de Asturias (PCTI-GRUPIN14-079). N.A-G. acknowledges a fellowship awarded by the SpanishMINECO (FPI program), and co-financed by the European Social Fund.

Author Contributions: N.A.-G. and M.M conducted experimental work. N.A.-G. and C.P. carried out dataanalysis and calculations. N.A.-G., M.V.G., F.R. and C.P. discussed the results and contributed to the writing ofthe manuscript.

Conflicts of Interest: Authors declare no conflicts of interest.

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